Question

# A common way of removing Cr3+ from solution is precipitation of Cr(OH)3(s). An environmental engineer tells...

A common way of removing Cr3+ from solution is precipitation of Cr(OH)3(s). An environmental engineer tells you that adjusting the pH to 8.5 will decrease the total soluble Cr(III) to below 1 mg/L. Is this true? In other words, is the solubility of Cr(III) in the presence of Cr(OH)3(s) at a pH of 8.5 less than 1 mg/L? the pKspof Cr(OH)3(s) is 30.22.

Note: This question is in 5th edition of Chemistry for Environmental Engineering and Science. Chapter 4, Question 86.

PH   = 8.5

POH = 14-PH

= 14-8.5 = 5.5

-log[OH-] = 5.5

[OH-]   = 10-5.5   = 3.16*10-6 M

PKsp     = 30.22

-logKsp = 30.22

Ksp   = 10-30.22    = 6.025*10-31

Cr(OH)3 -------> Cr+3 + 3OH-

Ksp   = [Cr3+][OH-]3

6.025*10-31 = [Cr3+]*(3.16*10-6 )3

[Cr+3]    = 6.025*10-31/(3.16*10-6)3

= 1.9*10-4 mole/L

= 1.9*10-4 * 52g/L

= 98.8*10-4g/L

= 9.88mg/L

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